The invention relates to interfaces for converting an incoming digital signal into a format for transmission on a synchronous digital network, to network elements comprising such interfaces, to corresponding receiver interfaces, to network elements having such interface, to corresponding methods and software, to methods of using data transmission services to cause data to be transmitted over such interfaces, and to methods of detecting transmission errors using such interfaces.
It is known to provide local area networks using protocols such as IEE 802.3, and ethernet (available in 10 megabits per second, 100 megabits per second and 1 gigabits per second versions), and to couple local area networks together to create wide area networks (WAN). Wide area networks often use the public telecommunications network. Conversion is required from LAN protocols to conventional telecoms interfaces, for example E1, E3, T1 and STM-1. ESCON (Enterprise Systems Connection) and Fibrechannel are further examples of known LANs or Storage Area Networks, for connecting multiple storage devices.
It is also known to connect LAN""s using optical transmission links, or optical transmission networks. There is a large installed base of SONET/SDH systems which can provide a transport service for ATM, SMDS, Frame Relay, T1, E1 and so on.
Mapping of one rate or format into another is well known. However, the standard or proprietary scheme allows transportation of a very specific set of signals, with format specific hardware. Generally separate hardware is required to map each type of signal onto SONET. It is known to map both continuous signals, which are synchronised to a clock, and burst format signals, which do not have a continuous clock. To transmit continuous signals, a wrapper is added to the continuous signal. However this produces formats which don""t have a pre-defined fixed bit rate. The resulting signal cannot be time multiplexed to be transported on a high speed network, otherwise the phase or synchronicity of the information is lost.
It has also been proposed to transmit LAN signals such as ethernet signals directly over a DWDM (Dense Wavelength Division Multiplexing) link without using a synchronous protocol such as SONET/SDH. This implies one of the wavelengths is dedicated to the LAN signal, as there is no way to multiplex other signals onto the same wavelength. This may leave the great majority of the bandwidth of the given wavelength unused, which may be unsatisfactory in some circumstances.
U.S. patent application Ser. No. 09/307812 (Solheim et al, entitled xe2x80x9cProtocol Independent-Rate Devicexe2x80x9d filed on May 10, 1999 and assigned to Nortel Networks Corporation) discloses a method of transporting different types of clients (IP, ATM, SONET, Ethernet etc) together. The bandwidth assigned to any given sub-rate channel can be provisioned without changing the hardware or software. U.S. patent application Ser. No. 09/349,087 (Roberts, entitled xe2x80x9cMapping Arbitrary Signals Into SONETxe2x80x9d, filed on Jul. 8, 1999 and assigned to Nortel Networks Corporation, ref. 10420RO), discloses mapping arbitrary signals into SONET to enable the signals to be recovered with low timing jitter at low cost. A mapper multiplexes numerous tributaries into the high rate SONET network. The mapper acts at the bit level to distribute stuffed bits uniformly interspersed across a frame, to enable an arbitrary input signal to be mapped onto the predefined fixed rate of the SONET/SDH output. This scheme and the above DWDM scheme both maintain inter frame information, and are both transparent to any frame format, meaning they are able to transport any frame format. However neither are frame aware and so have the disadvantage of not being able to carry out performance monitoring.
Other known schemes include encapsulation of frames for transmission, e.g. HDLC, (High-Level Data Link Control) and SDL(Simple Data Link), published by Lucent on the IETF web pages. The SDL publication is a proposal for encapsulating frames such as PPP (Point to Point Protocol) using SDL onto SONET/SDH. Such encapsulation schemes are frame aware and so can carry out performance monitoring. However, they have the disadvantages of not preserving information in the inter frame gaps, and of the mapping being specific to the frame format, so the schemes are not transparent.
It is also known to provide an interface between an ethernet network and a SONET/SDH system at a router or a bridge. In this case, the router or bridge may have interfaces dedicated to more than one LAN protocol, and may multiplex data on to the SONET/SDH system, but this involves recognising the layer 2/3 protocol which defines the contents of each frame or packet.
A disadvantage of such devices is the complexity of processing the layer 2/3 information, and the buffering of packets intended for various destinations. Accordingly, such devices are dedicated, and cannot handle frames or packets of an arbitrary layer 2/3 protocol.
It is an object of the invention to improve on the known schemes. According to a first aspect of the invention there is provided a sending interface for converting an incoming digital signal into a format for transmission on a synchronous digital network, the incoming digital signal having a group of bits coded by a predetermined line code, the incoming digital signal also carrying information for OSI layer 2 or 3 processing, the sending interface comprising:
circuitry for identifying the line code of the incoming digital signal, and
circuitry for carrying out the conversion of the incoming digital signal according to the line code identified, and independently of the information for OSI layer 2 or 3 processing.
This is the first time the advantages of performance monitoring capability and transparency have been possible together, as will now be explained. An advantage of identifying line codes is that it enables a greater degree of error detection and thus performance monitoring, compared to a bit based interface. This can be particularly significant if the interface is at a boundary between operating entities, such as a client/service provider boundary. Especially in such a case it can enable QoS (Quality of Service) to be offered and measured at a client/service provider boundary.
Another advantage of the conversion being line code aware, is that synchronisation can be simpler since line codes for padding can be added or deleted more easily, using lower specification hardware, than is needed for adding or subtracting bits. The interface can be semi-transparent in the sense that identification of line codes limits the interface to those formats that use identifiable line codes, but without limiting to a particular OSI layer 2 or 3 frame format.
Also, since OSI level 2 or 3 processing as carried out in a conventional router for example, is relatively complex, the interface of the invention can be greatly simplified and thus more easily integrated into other equipment, compared to the router for example. An advantage of the use of a synchronous digital network is that it facilitates multiplexing, and other transmission benefits.
Preferably the circuitry for identifying a line code comprises circuitry for identifying an idle code in the incoming digital signal. An advantage of this is that it enables the start and end of information streams such as variable length packets to be identified.
Preferably the circuitry for identifying a line code comprises circuitry for identifying a type of idle code, and the circuitry for carrying out the conversion is arranged to include in the synchronous data signal the type of idle code identified. An advantage is that information carried using different types of idle code will not be lost through the conversion.
Preferably the incoming digital signal comprises packets, and the circuitry for carrying out the conversion is arranged to replace one or more of the idle codes with a header for indicating the length of an associated one of the packets. This can enable a downstream receiver to identify the end of the associated packet, and thus identify idle codes, and maintain synchronicity with respect to packets and gaps between packets.
Preferably the header is of a fixed size. This can make synchronisation in the receiver easier.
Preferably the interface is arranged to adapt to receive incoming digital signals of more than one rate. An advantage is that the need for separate hardware and software for each rate is no longer needed. The adaptation could be automatic or carried out under the control of a network management system.
Preferably the format for the synchronous digital network comprises SONET/SDH virtual containers.
Preferably the interface comprises circuitry for carrying out virtual concatenation of the SONET/SDH virtual containers. In this specification, the term xe2x80x9cvirtual concatenationxe2x80x9d is used where the underlying network is unaware of any special relationship between the virtual containers which make up a group of virtually concatenated virtual containers. Particularly, although not exclusively, such frame based data may comprise OSI layer 2 data frames. An advantage is that delay variations between different paths in an SDH/SONET network can be handled.
Preferably the interface comprises a multiplexer for multiplexing more than one incoming digital signal onto the synchronous digital signal. An advantage is that bandwidth can be used more efficiently.
According to another aspect of the invention there is provided an interface for converting an incoming digital signal into a format for transmission on a synchronous digital network, the incoming digital signal having a series of packets, and a group of bits coded by a predetermined idle code separating the packets, the interface comprising:
circuitry for identifying the idle code of the incoming digital signal, and
circuitry for carrying out the conversion of the incoming digital signal according to the idle code identified.
According to a further aspect of the invention there is provided a receiver interface for recovering an incoming digital signal that has been converted to a signal of a format for a synchronous digital network, the incoming digital signal having a group of bits coded by a predetermined line code, the incoming digital signal also carrying information for OSI layer 2 or 3 processing, the receiving interface comprising:
circuitry for identifying linecode information in the formatted signal, and
circuitry for replacing the identified linecode information with corresponding linecodes independently of the information for OSI layer 2 or 3 processing.
An advantage is that this enables performance monitoring capability and transparency to be combined.
Preferably the interface comprises a retimer, for inserting or deleting one or more of the linecodes to match the incoming data rate to the required outgoing data rate.
Preferably the receiver interface is arranged to receive SONET/SDH virtual containers.
Preferably the receiver interface comprises circuitry for combining information from virtually concatenated containers before recovering the original incoming digital data signal.
Another aspect of the invention provides a corresponding method of, and corresponding software for converting an incoming digital signal into a synchronous digital signal.
Another aspect of the invention provides an SDH/SONET network element comprising the above interface.
Another aspect of the invention provides a system comprising the above receiving interface and circuitry for monitoring QoS performance.
Another aspect of the invention provides a method of using a data transmission service provided over a telecommunication network, comprising the step of causing data to be transmitted across the above interface.
Any of the preferred features may be combined with any of the aspects set out above as would be apparent to a skilled person.
Other advantages will be apparent to a skilled person, particularly in relation to any further prior art other than that discussed above.